The present invention concerns a circuitry by which at least two power control ranges can be established in a transmitter consisting of an amplifier of one or several stages.
In a transmitter of a radio telephone, class-C amplifiers are generally used, which are characterized by good efficiency, 60 to 80 per cent, on high power levels. Because of the great non-linearity of the amplifier, difficulties arise in realizing controlled power control on low power levels, and in addition, when low power levels are used, the efficiency of the amplifier reduces. This causes no harm when a system (or locality of use) is in question in which very low transmission power levels need not be used. The situation will be different, for instance in the digital GSM system covering in future all Europe, in which very low power levels will be in use. If a telephone is going to be used in an aeroplane, it is necessary to use, in order to prevent potential interferece to be caused in the aviational electronics, extremely low power levels.
A typical principle block diagram of a transmitter for a GSM radio telephone is presented in FIG. 1. The block diagram only shows the blocks required for understanding the operation. A signal to be transmitted enters e.g. the input RFin of a three-stage class-C amplifier 1. The gain of the amplifier 1 is controlled by a reference amplifier 3, the output of which is filtered before being input in the power amplifier 1. The input signals of the reference amplifier 3 are the voltage derived from a power detector 2, said voltage being proportional to the output voltage RFout of the power amplifier 1, and the control voltage TXC1 derived from the logic section of the telephone. Said blocks 1, 2 and 3 constitute a control loop which tends to be controlled in a state in which the voltage to be derived from the power detector 2 and the control voltage TXC1 derived from the logic sections of the radio telephone are of equal magnitude.
With the procedure of prior art, such power control range can be obtained which extends e.g. from +13 dBm to +33 dBm. If a wider power control range is wished, the following difficulties arise: the narrowness and non-linearity of the dynamics range and non-linearity of the power detector and the poor controllability of the power amplifier operating in class-C make the widening of the power range difficult.